1. Field of the Invention
The disclosed invention relates to aerogels and specifically to silica aerogels having three-dimensional nanoarchitecture with colloidal goldxcx9cprotein superstructures nanoglued therein. The disclosure describes methods for making the composite bioaerogels and their physical and chemical characteristics.
2. Description of Background Art
Much attention has been focused on immobilization of biomolecules in silicate glass formed by the sol-gel method, Eggers et al., Protein Sci. 2001, 10, 250-261. The process involves hydrolyzing an alkoxide to produce a sol, which then undergoes polycondensation to form a gel. Biomolecules are immobilized by being entrapped in the gel during the sol-to-gel transition. The sol-gel materials offer advantages over more traditional organic polymers for biomolecule entrapment in that these materials have increased mechanical strength, chemical stability, biocompatibility, and resistance to microbial attack.
While one can encapsulate a variety of biomolecules (enzymes, proteins, antibodies, cells) in sol-gel-derived matrices, the earliest reported bio/silicate-gels had only 30% activity as prepared using the conventional, alcohol-rich sol-gel preparation. Bioactivity of caged biomolecules rose to 75-95% upon the advent of the Dunn procedure, Dunn et al., Acta Mater. 1998, 46, 737-741, which uses less alcohol and provides better buffering of the sol. Traditionally when biomolecules have been incorporated into sol-gel-derived materials, the resultant gels are either kept wet (forming hydrogels) or are dried from aqueous conditions (forming xerogels) resulting in pore collapse of the material and long-sensing response times. The hydrogels are not ideal for real-world sensing either, in that they must be kept wet, stored at 4xc2x0 C., and the long-term stability of the encapsulated biomolecule has not been investigated. The longest reported lifetime of these materials is approximately a month when stored at 4xc2x0 C.
Heme proteins, such as horseradish peroxidase, Bhatia et al., Chem. Mater. 2000, 12, 2434-2441, cytochrome c (cyt. c) Lloyd et al., langmuir 2000, 16, 9092-9094 and myoglobin, Ellerby et al., Science 1992, 257, 1113-1116, have been extensively studied in sol-gel encapsulation. These proteins retain their spectroscopic properties and chemical functions of oxidation and reduction, ligand binding, or biocatalysis upon encapsulation. In one case, cyt. c was encapsulated into a sol-gel and absorbance-based spectral shifts were used to monitor binding of nitric oxide. Unfortunately, the sensor reaction is reported to have taken two hours to reverse, making dynamic measurements impractical, Aylott et al., Chem. Mater. 1997, 9, 2261-2263.
We have encapsulated heme protein into a silica framework, facilitated by formation of a proteinxe2x80x94protein superstructure nucleated in the liquid phase by colloidal gold. In solution, the protein is known to specifically adsorb via surface lysine residues to the surface of the Au with the heme pocket toward the metal. We posit that this adsorption-induced presentation of the back face of the protein to the protein-buffer medium leads to xe2x80x9ctailxe2x80x94tailxe2x80x9d-directed protein-protein assembly, alternating with xe2x80x9cheadxe2x80x94headxe2x80x9d protein-protein association, to leave essentially no unassociated protein in the buffered medium. The xe2x80x98outer skinxe2x80x99 of protein acts as a protective barrier and stabilizes the proteins within the superstructure against denaturants, including those arising during synthesis and processing of the silica nanoarchitecture. The goldxcx9cprotein-protein superstructure of the invention is nanoglued into the silica framework during the sol-to-gel transition. The wet gel is dried from supercritical fluid (SCF), forming a mesoporous aerogel, which permits true gasphase sensing with facile molecular transport into the biomolecule-modified three-dimensional nanoarchitecture of the aerogel. Although gold is described specifically throughout this disclosure, it is to be understood that silver, platinum, palladium, copper, and nickel may take the place of gold in this invention.